Signals subjected to magnetic recording and playback process are generally distorted because of the well known nonlinear frequency response of the recording/playback channel. Particularly when a digital signal is recorded and reproduced, it may be attenuated and distorted to such extent that the reproduced signal does not provide sufficient information about the originally recorded signal, nor does it provide distinct zero crossovers necessary for accurate detection of a self clocking signal. The signal distortion is due in part to the inductive nature of the playback head and to spacing losses. The inductance of the playback head prevents recovery of low frequencies in the signal played back from the recording medium. As it is well known, the playback signal is a function of the first time derivative of the recorded flux. Therefore, DC level and low frequency changes are not fully represented in the playback signal. On the other hand, spacing loss, which is a playback signal loss due to a nonzero distance between the playback head and the recording medium, contributes to high frequency attenuation. To compensate for both low and high frequency losses, it is generally known to use signal equalizers providing a substantially flat frequency response in the playback channel.
One known playback signal equalizer utilized in a digital magnetic recording/reproducing channel is described in the commonly assigned U.S. Pat. No. 4,110,798 to Miller et al. It has a low pass integrating circuit and a high pass differentiating circuit connected in parallel. The integrator provides an output signal which lags the input signal by 90.degree., and an output signal from the differentiator leads the input signal by 90.degree.. These respective output signals, which are equal in magnitude but of opposite polarity, are algebraically combined. The resulting equalized playback signal has a desired nearly flat frequency response.
The above-described type of equalizer has a disadvantage that the resulting equalized signal lacks DC and low frequency information. This leads to a non-constant average DC level, generally referred to as "baseline wander". The baseline wander is known to be caused by the loss of low frequency content due to the inductive nature of the playback head. The baseline wander appears as low frequency distortion in the equalized playback signal and it may cause misdetection by either shifting the zero crossing position of the bit detection, or by introducing a phase error in the clock synchronization circuit.
The above-described type of equalizer has a further disadvantage related to boosting the low frequency content of the playback data by the integrating channel. For example, in a high track density disk recorder the data detected by the playback head will contain low frequency information from the data track of interest, as well as low frequency crosstalk from adjacent tracks. Thus the equalizer will add unwanted cross talk into the equalized data.
Another type of playback equalizer is described in a publication Equalization Of The D. C. Null In High Density Digital Magnetic Recording, by W. D. Huber, paper 37-10, presented at the 1981 INTERMAG Conference, Grenoble, France, May 12 to 15. The equalizer of this publication partially restores the DC and low frequency content of a digital playback signal by centering the midpoint between the extreme peak amplitudes of the integrated signal. The centering is obtained by clamping and hard limiting the amplitude extremes of the integrated playback signal waveform to obtain positive and negative logic levels. However, the displacement of zero crossings due to baseline wander is not solved by this technique. The resulting processed waveform provides only a coarse representation of the recorded digital signal because it has noisy zero crossings and therefore unreliable clock recovery and data detection.